This invention relates to a method for producing a metallic bellows and an apparatus for carrying out the method. More specifically, this invention relates to a method for producing a metallic belows with a substantially oval cross-sectional shape and an apparatus for carrying out the method.
This bellows is used not only as a flexible joint and the like in fluid piping but, especially in a small size, as a cooling water passage for cooling a large-scale IC chip used in a large-scale computer and the like, which serves also as an elastic support for such chip.
In this specification, an "oval," "substantially oval," or "ovoidal" cross section includes not only a "mathematically or geometrically elliptical" shape but an "egg-shaped" configuration, and further includes also a profile at least one part of which, for example, the portion which is almost parallel to the major axis of an oval (hereinafter referred to as the "long side"), is linear. In this specification, noncircular ring forms are included in the term "oval" with the exception of polygonal shapes the vertexes or apexes of which are angular. In other words, a polygonal with all vertexes or apexes are rounded is included in the term "oval."
Also, in this specification, the term "bellows" means a tube whose side wall alternately provides ridges or large-diameter portions (bulged portions) and grooves or small-diameter portions (non-bulged portions). In a bellows, the cross-sectional size of all large-diameter portions may be the same or may not be the same, and that of all small-diameter portions also may be the same or may not be the same. Furthermore, the intervals of large-diameter portions may be the same or may vary in the longitudinal direction of the tube or bellows and those of small-diameter portions also may be the same or may vary in the longitudinal direction. Moreover, in a bellows, the cross-sectional shape of bulged portions may be or may not be substantially similar to that of nonbulged portions. The above-mentioned configurational conditions for a bellows can be selected conveniently depending on the elasticity, mechanical strength, etc. as in tension, compression and bending in various directions that the bellows is required to provide according to its application. When a bellows is used as a flexible joint in fluid piping and the like, the major or minor axis of the oval in the cross section of the bellows may be several millimeters to several centimeters or more in size. When a bellows is used as a cooling water passage or elastic support for a large-scale IC chip, the major or minor axis of the oval in the cross section of the bellows may be several millimeters or less in size.
Also, in this specification, the term "circular bellows" means a bellows with a circular cross-sectional shape and an "oval bellows" represents a bellows with a substantially oval or ovoidal cross section.
Furthermore, in this specification, "ovalizing" means transforming a cross-sectional form into an oval form.
Hitherto, bellows with an oval cross section have been produced, for example, as described in Japanese Patent Laid-Open No. 95927/86, from stock tubes with an oval cross section by means of a forming die that has protuberances formed in part of the semi-oval so that the portions of the ridges of the oval bellows to be formed which are virtually parallel to the major axis may be generally linear.
In this method of the prior art, however, expensive oval stock tubes are required and this may result in high production costs of oval bellows. Moreover, in the method disclosed in Japanese Patent Laid-Open No. 95927/86, the above-mentioned protuberances provided in the forming die cause a decrease in the cross section of the fluid passage at parts of the formed article including the inwardly protruded portions corresponding to the above-mentioned protuberances and no attention has so far been paid to this point. The inwardly protruded portions may also affect the fluid flow in the passage of the bellows to be disturbed.
Furthermore, when folding is conducted to form a bellows by axially applying compressive loads to the bulged part, the circumferentially non-uniform strain or deformation is caused to results in the circumferentially non-uniform stress remained in the bellows due to the inwardly protruded portions circumferentially locally formed in the grooves of the bulged part corresponding to the above-mentioned protuberances (which are formed in portions almost parallel to the major axis of the oval in terms of the cross section of the bulged part). Therefore, there is fear that the bellows thus produced is likely to be broken after repeated deformation in use.
When a stock tube with an oval cross section is bulged to form bulged portions as in this prior art and when the forming die is not provided with the protuberances disclosed in Japanese Patent Laid-Open No. 95927/86, portions of small curvature (i.e., large radius of curvature) that are virtually parallel to the major axis of the oval are radially bulged more than portions of large curvature that are virtually parallel to the minor axis upon bulging because of the oval shape of the cross section of the tube. The higher the ratio of the major axis to the minor axis of an oval, the greater the difference in the degree of this bulging. When a bellows is to be formed by axially applying compressive forces to a tube with bulged portions whose degree of bulging thus differs radially and by folding the bulged and nonbulged portions, buckling may occur in the portions of low degree of bulging (a case where the bulged amount is too small in the above-mentioned portions of large curvature) or excessive tensile deformation may occur in the portions of small degree of bulging (a case where the bulged amount becomes too large when an attempt is made to increase the bulged amount in the above-mentioned portions of large curvature). Therefore, necking or ductile fracture may occur in these portions.